The present invention relates to an apparatus for generating a magnetic field in a magnetooptic recording/reproducing system utilizing a magnetooptic effect and, more particularly, to an apparatus for generating a magnetic field for applying a magnetic field to an information recording medium to record or erase the information in a magnetooptic recording/reproducing system for recording or erasing the information on the medium by utilizing a magnetooptic effect.
In a magnetooptic recording/reproducing system utilizing a magnetooptic effect, a laser beam is converged onto an information recording medium (hereinafter referred to as "an optical disk") and a magnetic field is applied to a region on the optical disk irradiated with the laser beam in a recording or erasing mode. Therefore, the magnetooptic recording/reproducing system has an apparatus for generating a magnetic field for applying a magnetic field to the optical disk. A conventinal apparatus for generating a magnetic field is disclosed in Japanese Patent Laid-open Publication No. 119507/1984. The apparatus disclosed in this Publication has a magnetic yoke comprising a pair of bar sections and a base section on which the bar sections are formed, and the magnetic yoke has a length equal to the width of a region scanned with a laser beam in a radial direction of the optical disk. As shown in FIG. 1, in a magnetooptic recording/reproducing system, optical head 11 is disposed above one side surface of optical disk 10, and magnetic unit 13 is disposed under the surface of disk 10. In case of accessing a predetermined track on disk 10, head 11 is moved in the radial direction R of disk 10, as shown in FIG. 1 to record information on a desired track, to reproduce information from a desired track or to erase information recorded on a desired track. In a recording and reproducing mode, a vertical magnetic field is applied from unit 13 to disk 10. Unit 13 has bar sections 14B and 14C formed on base section 14A and an exciting coil 15 wound around one section 14B of magnetic yoke 14 which has a length equal to a width of a region to be scanned with a laser beam in a radial direction of disk 10.
Since a vertical magnetic field is applied to the entire region to be scanned with the laser beam in the radial direction of the optical disk by such magnetic unit 13, coil 15 increases in size and consumes a large amount of power and exciting coil 15 and its driving circuit generates a relatively large heat emission, thereby resulting in a high cost system. Since the vertical magnetic field is applied to not only the area irradiated with the laser beam but the other areas irradiated with no laser beam, another drawback arises that the efficiency of the magnetic field to be applied to the beam irradiated area per exciting power is wrong.
A magnetic unit for solving the above-mentioned drawbacks is disclosed in Japanese Patent Laid-open Publication No. 40761/1982. This magnetic unit 13 has, as shown in FIG. 3, one base section 14A having a length equal to a width to be scanned with a laser beam in a radial direction of an optical disk and a bar section 14B formed on the base section 14A, two segments 17A, 17B separated from each other and arranged along bar section on base section 14A, and exciting coils 18A, 18B respectively wound around divided segments 17A, 17B. A vertical magnetic field Hy having a level Hc or higher as shown in FIG. 4 is applied onto optical disk 10 by unit 13 in recording or erasing mode. In FIG. 4, when applying a vertical magnetic field H.sub.1 to first region Z.sub.1 of disk 10, first coil 18A is selected and merely excited. When applying a vertical magnetic field to second region Z.sub.2 of disk 10, second coil 18B is selected and merely excited. When applying a vertical magnetic field H.sub.3 to third region Z.sub.3 between regions Z.sub.1 and Z.sub. 2, one of coils 18A and 18B is selected and energized.
In unit 13 shown in FIG. 3, there arise drawbacks that a leakage magnetic flux occurs between segments 17A and 17B of yoke 14 as shown in FIG. 5 and intensity Hy of the vertical magnetic field decreases to level Hc or lower at the ends of regions Z.sub.1 and Z.sub.2 as shown in FIG. 4. As a result, intensity Hc of a magnetic field sufficient to record or erase information is not applied to a boundary region on disk 10 facing a space region between segments 17A and 17B, and information may not be stably recorded nor reliably erased. In order to solve this problem, it is considered that a relatively large current should always be supplied to the selected coil to generate a vertical magnetic field having sufficient intensity Hc. Since a large current must be supplied to the coil, the result is a large amount of heat generated in the coil and large power consumption.